This invention relates to a method of manufacturing potassium sulfate, particularly of the type applied as an artificial fertilizer, by reaction between potassium chloride and sulfuric acid.
Potassium chloride is widely used as a potash fertilizer. In Japan, about one million tons per year of potassium chloride are consumed, and the total quantity thereof is imported from abroad. Though more expensive than potassium chloride, potassium sulfate is more preferably applied, because the presence of chlorine ion in the soil is detrimental to the production of tobacco and potato or other horticultural plants. Further, where a compound fertilizer is manufactured, the mixture of potassium chloride with ammonium nitrate tends to give rise to an explosion. For these reasons, potassium sulfate is used partly instead of potassium chloride. In Japan, about three hundred thousand tons per year of potassium sulfate are consumed. Substantially the whole quantity of said potassium sulfate is imported from abroad, and a very small amount of potassium sulfate is domestically manufactured.
Manufacture of fertilizer type potassium sulfate from raw potassium chloride has long been carried out by the process of mixing a chemically equivalent amount of concentrated sulfuric acid with raw potassium chloride, and heating the mixture for many hours at a high temperature of 500.degree. to 600.degree. C. to cause thermal decomposition expressed by the following chemical equation (1). ##EQU1## Thus, agglomerate potassium sulfate and gaseous hydrogen chloride can be obtained.
The above-mentioned process is a classic one closely resembling the former stage of the Leblanc soda process developed about 100 years ago.
This process has a drawback that the reaction apparatus is tremendously corroded by the high temperature hydrogen chloride gas released from the reaction. Further, heat conduction from the muffle furnace to the mixture of raw materials is not good so that the reactor must occupy a large volume and the mechanical operation of the furnace becomes very difficult. Therefore, this process leads to heavy losses in thermal and mechanical energy, unfavorably consuming considerable plant and operation costs.
The solid phase reaction applied in this process proceeds at a lower velocity than the reaction in an aqueous solution, making it very difficult to reduce the content of impurity. By this process, it is ordinarily impossible to reduce the content of residual chloride ion content to less than 3 weight% in the produced potassium sulfate. If it is attempted to decrease the quantity of residual chloride ion to about 1 weight%, the reaction temperature should be raised up to about 800.degree. C., resulting in far greater difficults.
Heretofore, a number of inventions have been developed in order to improve the process of manufacturing potassium sulfate from potassium chloride and concentrated sulfuric acid. These inventions can be classified definitely into two kinds, that is, a completely dry thermal decomposition process and a completely wet double decomposition process.
An improvement on the aforementioned classical dry process is set forth in Japanese Patent Application Publication No. 2666/1957. According to this invention, the reaction expressed by the chemical equation (1) is undertaken in the following two stages expressed by the equations (2) and (3) given below. EQU nKCl+H.sub.2 SO.sub.4 =K.sub.n H.sub.2-n SO.sub.4 +nHCl.uparw.(2) EQU K.sub.n H.sub.2-n SO.sub.4 +(2-n)KCl=K.sub.2 SO.sub.4 +(2-n)HCl.uparw.(3)
(where n is taken to denote 1.3 to 1.4)
The dry thermal reaction of the first stage (2) terminates in 1 to 3 hours with the temperature kept at a low level of about 250.degree. C. A semimolten mass obtained by the reaction (2) is solidified by cooling, and then the mass is crushed into powdery particles having a size of about 60 mesh. Afterwards, the second stage (3) of the dry thermal reaction is finished in 1 to 3 hours with the temperatue kept at a level ranging from 400.degree. to 500.degree. C.
The above-mentioned improved process is characterized in that a lower reaction temperature is applicable than in the previously described classical one stage process based on the equation (1) as a whole, and the content of residual chloride ion in the product can be somewhat reduced. However, this improved process is still accompanied with the drawbacks that a semimolten mass should be solidified by cooling when the first stage reaction is shifted to the second stage reaction, and the solidified mass should be finely pulverized with great difficulties. The pulverized mass has to be heated again to a high temperature, and tends to be fused into an agglomerated mass. Thus, this developed process offers little advantage over the aforementioned classical process.
The process set forth in Japanese published unexamined patent application No. 90488/1980 is characterized in that, when the dry one stage reaction of the equation (1) is carried out, the concentrated sulfuric acid is applied at such an excessively large equivalent ratio as 1.07 to 1.40 times (preferably 1.1 to 1.3 times) the content of potassium chloride. By this process, the reaction can be finished in about 2 hours with the temperature kept at a relatively low level of about 400.degree. C., obtaining apparently dry product of potassium sulfate containing acid potassium sulfate. Further, the product contains residual chloride ion of less than 1 weight%.
The product of this process, however, contains an excess amount of sulfuric acid, making the product acidic. This acidic product can not be used as such as a fertilizer. The acidic product can be used only as a raw material of compound fertilizer production. If the acidic product is neutralized by a common alkaline agent, the neutralized product can be used as a fertilizer. However, the application of an expensive alkaline agent tends to raise the cost of the product, and moreover undesirably reduces its purity.
The processes disclosed in U.S. Pat. Nos. 2,706,145 and 3,563,701 are characterized in that the muffle furnace is replaced by a specially designed fluidized bed furnace having a high thermal efficiency. After all, these processes are nothing but the aforesaid one stage dry reaction type. Moreover, the products of these processes have an unsatisfactory purity ranging from 95 to 97%. Further, these processes are indeed applicable to the manufacture of sodium sulfate, but are unadapted for the manufacture of potassium sulfate, because the fluidized bed of the latter tends to coagulate.
Next, with regard to wet process, U.S. Pat. No. 4,045,543 comprises the steps of reacting 2 mols of KCl with 1 mol of H.sub.2 SO.sub.4 in a medium of water at a temperature of 65.degree. to 120.degree. C. in accordance with the undermentioned chemical equation of (4), EQU 2KCl+H.sub.2 SO.sub.4 =K.sub.2 SO.sub.4 +2HCl (4)
evaporating the produced HCl in the form of an azeotropic mixture with water at a temperature of 90.degree. to 110.degree. C., replenishing the same amount of water as that which has been evaporated, and cooling the resultant solution to crystallize out a product of K.sub.2 SO.sub.4. A large amount of potassium salts remain in the solution, and are circulatingly returned to the starting reaction vessel.
However, it has been proved by the present inventors that it is necessary for the above-mentioned process to evaporate such a large amount of water which equals about 9 times the weight of the potassium sulfate product. Obviously, the above-mentioned wet process consumes a far larger amount of fuel than the conventional dry process, not being practically applicable from the economic point of view.
The process set forth in Japanese Patent Application Publication No. 27,246/1967 resembles to that of U.S. Pat. No. 4,045,543 mentioned above. In this process, it has been also proved by the present inventors that it is necessary to evaporate such a large amount of water which equals from about 6 to 11 times the weight of the potassium sulfate product, even if the filtrate is circulatingly returned to the starting reaction vessel.
An error common to the literature of the above-mentioned wet processes arises from the undermentioned illusion. Namely, when a filtrate is concentrated or evaporated, water and hydrochloric acid constitute jointly an azeotropic mixture as in well known. Therefore, it is impossible to separate hydrochloric acid from water by evaporating an aqueous solution of hydrochloric acid. Further, when hydrochloric acid is contained in the solution at a low concentration, substantially steam alone is vaporized, resulting in the extremely small evaporation recovery of hydrogen chloride. On the other hand, when hydrochloric acid is contained in the solution at a high concentration, potassium sulfate and hydrogen chloride contained in the solution give rise to a backward reaction and tend to go back to acid potassium sulfate and potassium chloride. Therefore, hydrochloric acid itself does not indicate so high a partial pressure as does a simple aqueous solution of hydrochloric acid. Even if, therefore, all the content of water is evaporated, it is impossible to recover the whole of the hydrochloric acid which is produced.
Upon ignoring the above-mentioned facts, it has been erroneously believed that, when the recovered filtrate is thermally concentrated, the hydrochloric acid contained in the filtrate can be easily evaporated off. It is actually possible to evaporate off less than 25% of the total of the hydrogen chloride obtained. In all the aforementioned literature of wet process, there is no distinct description of a material balance, concerning the circulating system of recovered filtrate.
As mentioned afore, the present inventors have proved that all the wet processes cited above consume an extremely large amount of fuel used for evaporation of water in the filtrate, and are practically inapplicable from the economical point of view. To date, these wet processes have not been practised in the world. Only the dry process is used.